Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide, and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone.
Another process involves the catalytic hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513, in which the hydroalkylation catalyst is a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family.
However, one problem in producing phenol via cyclohexylbenzene is that the oxidation of cyclohexylbenzene is considerably more difficult than that of cumene. Thus, whereas cumene oxidation is normally conducted in the absence of a catalyst, cyclohexylbenzene oxidation typically requires the presence of a catalyst containing a nitroxyl radical, particularly a cyclic imide, such as N-hydroxyphthalimide (NHPI), to provide commercially acceptable levels of conversion. Moreover, even using NHPI as a catalyst, the selectivity to cyclohexylbenzene hydroperoxide decreases with increasing conversion. Also, although the oxidation of cyclohexylbenzene is exothermic, the temperature must be controlled within a relatively narrow range if the production of unwanted by-products is to be minimized. Thus, there is significant interest in developing improved methods of oxidizing cyclohexylbenzene that allow for increased yields of the desired hydroperoxide.
According to the present invention, it has now been found that an advantageous combination of high conversion and high selectivity can be achieved in the oxidation of cyclohexylbenzene in the presence of a cyclic imide catalyst by conducting the oxidation in a plurality of series-connected reactors with the temperature being decreased and/or the catalyst concentration being increased from the first to the final reactor.
U.S. Pat. Nos. 6,852,893 and 6,720,462 describe methods for producing phenol by catalytic oxidation of alkylaromatic hydrocarbons to the corresponding hydroperoxide, and subsequent cleavage of the hydroperoxide to give phenol and a ketone. Catalytic oxidation takes place with oxygen, in the presence of a free radical initiator and a catalyst, typically an N-hydroxycarbodiimide catalyst, such as N-hydroxyphthalimide. Preferred alkylaromatic hydrocarbons that may be oxidized by this process include cumene, cyclohexylbenzene, cyclododecylbenzene and sec-butylbenzene.
International Patent Publication No. WO2010/074779 discloses a process for oxidizing an alkylaromatic compound, such as sec-butylbenzene and cyclohexylbenzene, to the corresponding alkylaromatic hydroperoxide by contacting the alkylaromatic compound with oxygen in the presence of a cyclic amide catalyst, such as N-hydroxyphthalimide, which is substantially free of alkali metal compounds. According to this publication, if the oxidation is conducted at a temperature of about 90° C. to about 150° C. with the cyclic imide being present in an amount between about 0.05 wt % and about 5 wt % of the alkylaromatic in the feed, conversion rates of at least 4 wt %/hour can be achieved with a selectivity to the hydroperoxide of at least 90%.